ASC is an activating adaptor for NF-kappa B and caspase-8-dependent apoptosis

ASC is a pro-apoptotic protein containing a pyrin domain (PD) and a caspase-recruitment domain (CARD). A previous study suggests that ASC interacts with Ipaf, a member of the Apaf-1/Nod1 protein family. However, the functional relevance of the interaction has not been determined. Here, we report that co-expression of ASC with Ipaf or oligomerization of ASC induces both apoptosis and NF-kappa B activation. Apoptosis induced through ASC was inhibited by a mutant form of Caspase-8 but not by that of Caspase-1. The PD of ASC physically interacted with Caspase-8 as well as with pyrin, the familial Mediterranean fever gene product. Caspase-8 deficiency rescued mouse fibroblasts from apoptosis induced by ASC oligomerization. Pyrin disrupted the interaction between ASC and Caspase-8, and inhibited both apoptosis and NF-kappa B activation induced by ASC. These findings suggest that ASC is a mediator of NF-kappa B activation and Caspase-8-dependent apoptosis in an Ipaf signaling pathway.     

Crystal structure of a catalytic site mutant of beta-amylase from Bacillus cereus var. mycoides cocrystallized with maltopentaose

The X-ray crystal structure of a catalytic site mutant of beta-amylase, E172A (Glu172 --> Ala), from Bacillus cereus var. mycoides complexed with a substrate, maltopentaose (G5), and the wild-type enzyme complexed with maltose were determined at 2.1 and 2.0 A resolution, respectively. Clear and continuous density corresponding to G5 was observed in the active site of E172A, and thus, the substrate, G5, was not hydrolyzed. All glucose residues adopted a relaxed (4)C(1) conformation, and the conformation of the maltose unit for Glc2 and Glc3 was much different from those of other maltose units, where each glucose residue of G5 is named Glc1-Glc5 (Glc1 is at the nonreducing end). A water molecule was observed 3.3 A from the C1 atom of Glc2, and 3.0 A apart from the OE1 atom of Glu367 which acts as a general base. In the wild-type enzyme-maltose complex, two maltose molecules bind at subsites -2 and -1 and at subsites +1 and +2 in tandem. The conformation of the maltose molecules was similar to that of the condensation product of soybean beta-amylase, but differed from that of G5 in E172A. When the substrate flips between Glc2 and Glc3, the conformational energy of the maltose unit was calculated to be 20 kcal/mol higher than that of the cis conformation by MM3. We suggest that beta-amylase destabilizes the bond that is to be broken in the ES complex, decreasing the activation energy, DeltaG(++), which is the difference in free energy between this state and the transition state.     

Crystal structures of beta-amylase from Bacillus cereus var mycoides in complexes with substrate analogs and affinity-labeling reagents

The crystal structures of beta-amylase from Bacillus cereus var. mycoides in complexes with five inhibitors were solved. The inhibitors used were three substrate analogs, i.e. glucose, maltose (product), and a synthesized compound, O-alpha-D-glucopyranosyl-(1-->4)-O-alpha-D-glucopyranosyl-(1-->4)-D-xylopyranose (GGX), and two affinity-labeling reagents with an epoxy alkyl group at the reducing end of glucose. For all inhibitors, one molecule was bound at the active site cleft and the non-reducing end glucose of the four inhibitors except GGX was located at subsite 1, accompanied by a large conformational change of the flexible loop (residues 93-97), which covered the bound inhibitor. In addition, another molecule of maltose or GGX was bound about 30 A away from the active site. A large movement of residues 330 and 331 around subsite 3 was also observed upon the binding of GGX at subsites 3 to 5. Two affinity-labeling reagents, alpha-EPG and alpha-EBG, were covalently bound to a catalytic residue (Glu-172). A substrate recognition mechanism for the beta-amylase was discussed based on the modes of binding of these inhibitors in the active site cleft.     

Identification of a monofunctional aspartate kinase gene of Arabidopsis thaliana with spatially and temporally regulated expression

We screened a gene trap library of Arabidopsis thaliana and isolated a line in which a gene encoding a homologue of monofunctional aspartate kinase was trapped by the reporter gene. Aspartate kinase (AK) is a key enzyme in the biosynthsis of aspartate family amino acids such as lysine, threonine, isoleucine, and methionine. In plants, two types of AK are known: one is AK which is sensitive to feedback inhibition by threonine and carries both AK and homoserine dehydrogenase (HSD) activities. The other one is monofunctional, sensitive to lysine and synergistically S-adenosylmethionine, and has only AK activity. We concluded that the trapped gene encoded a monofunctional aspartate kinase and designated as AK-lys3, because it lacked the HSD domain and had an amino acid sequence highly similar to those of the monofunctional aspartate kinases ofA. thaliana. AK-lys3 was highly expressed in xylem of leaves and hypocotyls and stele of roots. Significant expression of this gene was also observed in trichomes after bolting. Slight expression of AK-lys3 was detected in vascular bundles and mesophyll cells of cauline leaves, inflorescence stems, sepals, petals, and stigmas. These results indicated that this aspartate kinase gene was not expressed uniformly but in a spatially specific manner.     

Effect of epidermal growth factor on intestinal adaptation after allogeneic small bowel transplantation in rats

We reported that epidermal growth factor (EGF) stimulated graft adaptation in a rat model of syngeneic small bowel transplantation. However, graft rejection is a severe problem with clinical small bowel transplantation, because small intestinal wall contains large amounts of lymphoid tissue. Studies were performed to investigate the effect of EGF on allogeneic graft adaptation after small bowel transplantation in rats treated with an immunosuppressant FK506. The transplanted animals received intraperitoneally EGF or saline (untreated) after surgery and were examined for analysis one week later. EGF-treated group markedly enhanced the water absorption and induction of sodium glucose cotransporter (SGLTI) as compared with EGF-untreated group. EGF-treated group also increased the mucosal crypt depth and its cell proliferating rate, although there was no significant difference in the mucosal villus height between the two groups. These results indicate that EGF accelerates intestinal allograft adaptation in part by the recovery of mucosal structure and function after small bowel transplantation in rats. EGF may have relevance to promote graft function in clinical small intestinal transplantation.